1. Field of the Invention
The present invention is directed to catalysed filters made of bio-soluble fibers.
2. Description of the Related Art
A combination of a dust filter and a catalyst is highly desired in the industry where both particle removal and gaseous emission control are required in order to meet the local regulations.
Examples are combustion processes for the power production where both fly ash, nitrogen oxides (NOx) and CO may have to be removed. For instance, removal of NOx can be effectively achieved by the SCR process which involves injection of NH3 and reaction of NOX on a V-based catalyst.
Combining a dust filter and a catalyst gives the possibility to have two or more processes combined in one physical installation. The footprint of the required after-treatment installation is then decrease drastically. Furthermore, the operation of the plant requires fewer operators.
The combination of catalysts and filters is, however, not as straightforward. Catalysts may importantly differ in chemical composition depending on the particular application. Both the support and the active phase may be constituted by several chemical elements. Filters may also be constituted by different chemical compounds, which in many cases may interfere with the catalyst activity. In some cases, the interaction between the filter constituents and the catalyst may cause poisoning of the catalyst and subsequently poor performance of the catalytic filter.
In the case of high temperature ceramic filters, for instance, several types of fibers may be used for their production. These can be constituted e.g. by silica-aluminate, calcium-magnesium-silicates, calcium-silicates fibers, or a mixture of them, all containing alkali and/or alkali-earth metals in different quantities as constituents.
Another way of distinguishing between the different ceramic filters is by classifying their fibers as bio-soluble or not. Bio-soluble fibers are preferred since they have a high solubility in physiological media and do not pose risks to the health of humans. Non bio-soluble fibers are instead being classified as carcinogenic. Silica-alumina fibers are an example of non-bio-soluble fibers.
Today, the only ceramic catalytic filters available on the market are based on non-bio-soluble silica-aluminate fibers and V/Ti catalyst for the simultaneous removal of dust, NOx and dioxins. In particular, the combination of the silica-aluminate fibers and the V/Ti catalyst ensures no poisoning of the catalyst. These fibers are chemically very stable and no poisons are released during operation, which, in the case of the SCR reaction is in the range 180-370° C. Despite their stability and high efficiency, there is the wish of using ceramic catalytic filters for the removal of NOx based on the same V/Ti catalyst but on bio-soluble fibers. We have attempted developing such a filter and observed that most of the bio-soluble fibers tend to deactivate the V/Ti catalyst. The reason for the deactivation has been found in the alkali-content found in the bio-soluble fibers. Alkali metals (i.e. K and Na) are in fact very strong poisons for the V-based SCR catalyst. In most of the tested fibers, the alkali fraction is not strongly bound to the fiber constituent, so that, during operation some alkali metals are released and get into chemical contact with the catalyst particles.